Look at the shape of the leaves. Poison ivy leaves are wide, spoon-shaped leaves.
Count the leaves. Poison ivy leaves usually grow up clumps of three, although occasionally there are more leaves than that.
Look at the color of the leaves. In summer, poison ivy leaves are shiny green. In autumn, the leaves turn red.
Look for berries in the autumn. In autumn, poison ivy plants have small, cream-colored berries.
Understand that poison ivy can grow on vines or shrubs. In the eastern U.S., it most often grows as a low vine. In the northern U.S. and in Canada, it most often grows as a shrub.
Consider where you are. Poison ivy is found all over the U.S. except in Alaska and Hawaii. However, it requires a fair amount of moisture to grow. It is often found along creeks or riverbanks. You won't find it in the dessert, unless there is a river nearby or there has been a lot of rain lately.